6:00 p.m. update: We can signal the all-clear in terms of thunderstorms for the D.C. metro region – and expect the severe thunderstorm watch will soon be dropped. While the storms split the Beltway area, we received multiple reports of hail in from western Howard County through Columbia and into Laurel. And a possibly tornadic storm tracked through northern and central Anne Arundel County – but there has been no tornado confirmation. Strong storms also tracked south of the District through Stafford County and southern Maryland. In Solomons (Charles County), golf ball-size hail was reported.

5:10 p.m. update: The tornado-warned storm over Anne Arundel has moved over the Bay. The coast is now clear.

5:00 p.m. update: The tornado warning remains in effect for Anne Arundel County until 5:15 p.m. Doppler radar indicates the possibility of a tornado between Shady Side and Deale. Storm should be moving out over the Bay in the next few minutes.

4:55 p.m. update: From the National Weather Service at 4:47 p.m.

A SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO WAS LOCATED NEAR LONDONTOWNE…OR 5 MILES NORTHWEST OF RHODE RIVER… AND WAS MOVING EAST AT 35 MPH.

4:50 p.m. update: Here’s a radar view of the hook echo, which may indicate a tornado in central Anne Arundel County. Based on radar, this would pass south of Annapolis and could impact areas just west and west northwest of Shade Side over the next 15 to 20 minutes.

4:35 p.m. update: So far, storms have totally done a D.C. split- with strong cells north and south of the District and nothing near the Beltway. In addition to the strong storm in northern and central Anne Arundel County, the other biggie is straddling Stafford and western Charles County – where a severe thunderstorm warning is in effect until 5:15 p.m.

4:30 p.m. update: Storm that produced hail around Columbia and Laurel now into Odenton and Crofton and headed into Annapolis over next half hour.

4:19 p.m. update: Multiple reports of hail from the storm that pushed through Laurel.

3:48 p.m. update: Western Howard County is now bearing the brunt of the severe storm issued just before 3:30 p.m. The storm has a history of producing some hail and will strike Columbia in the next 15 minutes.

3:14 p.m. update: Storm are rapidly developing to the northwest and south of the District. They tend to be widely scattered but are gradually increasing in coverage and intensity. The strongest cell is near Frederick rapidly moving east/southeast into western Howard and extreme northern Montgomery counties in the next 30 minutes. Most of the other action is in eastern Fairfax and Prince William counties rapidly crossing the Potomac towards southwest Prince George’s and western Charles counties. No severe warnings…yet.

Storms that develop should be fast moving and the most likely window for storms is fairly short – essentially over the next 2-3 hours mostly.

2:24 p.m. update: A severe thunderstorm watch has been issued for the Washington, D.C. and Baltimore regions through 10 p.m. The National Weather Service’s Storm Prediction Center says: THUNDERSTORMS ARE EXPECTED TO DEVELOP THIS AFTERNOON ACROSS THE MOUNTAINS OF WESTERN VA…SPREADING SOUTHEASTWARD ACROSS THE WATCH AREA THIS AFTERNOON AND EVENING. ORGANIZED MULTICELL STORMS CAPABLE OF HAIL AND DAMAGING WINDS ARE POSSIBLE. It notes there’s a 50 percent or “moderate” chance of at least 10 occurrences of damaging winds as well as large hail in the watch region (shown to the right above). Remember that a watch means the environment is favorable for development of severe storms but no guarantee. It’s essentially a yellow light: use caution. A warning – on other hand – is effectively a red light: it means severe storms are occurring or imminent and you should seek shelter.

Overview from 2:05 p.m.: Unlike yesterday’s severe thunderstorm threat, today our entire region has received strong solar heating, and the atmosphere has become quite unstable. Where will any strong to severe thunderstorms likely fire later this afternoon? There’s a chance of some nasty storms – particularly between 3 and 6 p.m. in the immediate D.C. area and to the east. The National Weather Service Storm Prediction Center (SPC) is contemplating issuing a severe thunderstorm watch for the region. Let’s dig a little deeper.

Quick overview

Figure 1 below highlights the region under a slight risk for severe thunderstorms this afternoon, as issued by NOAA’s Storm Prediction Center (SPC) at 1230 p.m. The greater D.C. region is barely tucked within the western border of greatest likelihood, with highest probability (15% for damaging wind gusts and large hail, less than 1% chance of a tornado) along and east of I-95.

The large-scale surface weather chart (Figure 2) shows that we remain in the warm sector of a slowly-moving, weakening storm system (the low pressure center is located well to our north, in Canada). A weak cold front is slowly moving eastward. A weak wave of low pressure has developed along the warm front over the Delmarva, and a lee trough extends along the eastern shore.

In the upper atmosphere, all is quiet; there are no apparent upper-level disturbances approaching the region from the Ohio Valley. Since sunrise, lack of cloud cover has enabled very strong solar heating, and combined with decent surface moisture (especially in regions that received a good soaking in the past 12 hours), the atmosphere has become quite unstable. A measure of instability is the convective available potential energy (CAPE) – that is, a reservoir of buoyancy that thunderstorm updrafts can tap. The distribution of CAPE at 12:30 this afternoon is shown in Figure 3. Values exceed 2000 J/kg across much of our region, which is a moderate value.

Finally, like yesterday, wind shear is robust across the region as well. This is a measure of how fast the winds increase with altitude. Today, we have values of 40 knots through a deep layer. Wind shear increases the intensity of storm updrafts and downdrafts, and if strong enough can promote storm rotation – leading to mini-supercells and bowed line segments (bow echoes).

What will happen, where and when?

Given an axis of strong instability lying along the I-95 corridor, what we need is a means to lift the air, triggering cloud updrafts. Today, that lift could come from one of several processes. Certainly one of these is along the approaching cold front. Another may be along the southward bulge along the stationary front in the vicinity of Baltimore, across which there is strongly converging flow. The mountains are another possible location to storm initiation, as has already been occurring over Pa. Yet another location might be along the lee trough cutting through southern Maryland.

We often look to the high resolution model runs to give us guidance on where storms might initiate and intensify. The morning’s high-resolution NAM run is shown below (Figure 4). It suggests that a couple regions of storms will develop ahead of the frontal bulge and along the lee trough between 3-4 pm, then move toward the southeast.

Another high res model, the HRRR, is predicting a couple broken lines of storms will develop along I-95 and race toward the southeast, starting around 3-4 p.m. The WRF-ARW favors a cluster of development mainly over the Delmarva in the 4-6 p.m. timeframe.

Thunderstorms will mostly likely initiate both along the eastern slopes of the mountains and along the I95 region, starting as early as mid-afternoon. As these storms move southeast, they may intensify, shifting their most vigorous phase east of the immediate D.C.-Baltimore corridor, during late afternoon-evening.

The most likely mode of organization will be clusters/short lines of multicell-type storms. Multicells are larger and longer lived than single-cell “pop up” storms. It’s possible that a few could acquire small bow-echo characteristics. Because of their fast movement and lower precipitable water today (compared to yesterday), flash flooding is not a big concern with these storms. These storms however could produce very intense lightning, marble hail and damaging wind gusts.

A couple of mitigating factors

There are two caveats today, that might limit coverage and intensity of storms.

The first: A weak jet streak (pocket of fast wind) is approaching our region from the Ohio Valley. While this will increase wind shear area-wide this afternoon, our region lies beneath a specific portion of that wind pocket known to generate sinking air. Sinking air, in turn, suppresses deep cloud growth and tends to dry out the lower atmosphere.

The second: The area of weak low pressure to our east, as well as the lee-side trough of low pressure, could shift the low-level winds to northwesterly this afternoon. When air is forced to descent the mountains, it tends to dry out, and a surface northwest flow-regime in general does not favor widespread growth of thunderstorms. Either, or both, of these factors could act to “thin out” convective growth area-wide, leading to an underwhelming risk scenario.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.

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